1. Field of the Invention
The present invention generally relates to a flow sensor for measuring a flow speed or flow rate of a fluid such as, for example, flow rate of an intake air in an internal combustion engine system or the like. More particularly, the present invention is concerned with an improved structure of a flow-rate detecting device for the flow sensor which device includes a heating element for measuring the flow speed or flow rate of a flowing fluid on the basis of phenomenon of heat transmission or heat transfer from the heating element to the flowing fluid.
2. Description of Related Art
For better understanding of the concept underlying the present invention, description will first be made of the hitherto known or conventional flow-rate detecting device for the flow sensor by reference to FIGS. 14 and 15 of the accompanying drawings, in which FIG. 14 is a top plan view showing a conventional flow-rate detecting device of diaphragm type which is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 249693/1994 (JP-A-6-249693), and FIG. 15 is a sectional view of the same taken along a line Gxe2x80x94G and viewed in the direction indicated by arrows in FIG. 14.
Referring to FIGS. 14 and 15, the flow-rate detecting device generally denoted by 12 includes a heating element 51 formed on a membrane or film 53 and a fluid temperature measuring element 52 formed on a membrane or film 54. Both the films 53 and 54 are supported under tension on a frame 56 which is made of single crystal silicon. Disposed between the films 53 and 54 is a supporting member 58 formed of silicon which serves as an isothermal element (see FIG. 15). To this end, the frame 56 is made of a silicon plate 57 which is formed or pierced with a pair of through-holes 60 each of trapezoidal form in vertical section as viewed in FIG. 15. These through-holes 60 of trapeziform in section extend to a film layer 59 which is formed on a top surface (as viewed in FIG. 15) of the silicon plate 57, whereby a pair of diaphragm structures are implemented. Further, terminals 55 are provided on the frame 56 for electrical connection to an external circuit of the fluid sensor.
The conventional flow-rate detecting device 12 implemented in the diaphragm structure as described above suffers a problem that solid particulates or particles such as sands, dusts, etc. carried by the measurement-destined fluid (i.e., fluid whose flow rate or speed is to be measured) collide with the diaphragms which may thereby be physically broken or fractured. For coping with this problem, the mechanical strength of the diaphragms may be increased by increasing the thickness of those portions of the film layer 59 which correspond to the diaphragms, respectively. However, in that case, sensitivity of the flow sensor will become lowered due to the increased thickness of the film layer 59, which results in degradation of the response performance or characteristic of the flow sensor, giving rise to another problem.
In general, a supporting film and a protection film which constitute parts of the film layer 59 are formed by controlling the conditions for film formation such that a predetermined tensile stress is applied so that the diaphragm can be protected against deformation or distortion even when the heat generating resistance film which serves as the heating element undergoes thermal expansion. More specifically, when the diaphragm is deformed, large stress makes appearance between the diaphragm and the heat-sensitive resistance film which serves as the temperature detecting element, as a result of which delamination takes place between the heat-sensitive resistance film and the diaphragm, exerting adverse influence to the detection performance of the flow-rate detecting device and hence of the flow sensor. Furthermore, when remarkably large deformation has occurred, dispersion or variance will take place in the film deformation due to dispersion or variance of the thermal and/or mechanical properties (physical properties) of the film, which involves adverse influence to the detection output characteristic of the flow sensor, making thus it difficult or impossible to realize the flow-rate detection with acceptable accuracy and reliability.
Further, when the tensile stress mentioned above is set excessively high, the margin for the breaking stress under which the diaphragm is fractured becomes narrower, as a result of which the diaphragm becomes more likely to be fractured. For the reason mentioned above, it is desirable to set the internal stress of the diaphragm such that no deformation of the diaphragm can occur under the tensile stress set as low as possible even when the heating resistor undergoes thermal expansion.
In the environment in which the flow sensor is ordinarily employed, the internal stress of the diaphragm provided with the heating element (formed of a heat generating resistance film) is set lower than the diaphragm which is provided with the fluid temperature measuring element (also formed of a heat-sensitive resistance film) in view of the thermal expansion which the heat generating resistance film undergoes. Accordingly, the margin of the diaphragm provided with the fluid temperature measuring element to the fracture stress is narrower than the diaphragm provided with the heating resistance element and thus the former is more likely to be fractured when compared with the latter.
In the light of the state of the art described above, it is an object of the present invention to solve the problems of the conventional flow sensor and provide an improved structure of a flow-rate detecting device for a heat-sensitive type flow sensor which device can ensure an enhanced reliability for flow-rate detecting operation owing to the increased strength of a diaphragm structure as a whole by providing a reinforcing film for a diaphragm portion in which a fluid temperature measuring element is incorporated and which is thus more susceptible to fracture.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a flow-rate detecting device for a heat-sensitive type flow sensor, which device includes a planar substrate having first and second through-holes formed therein in juxtaposition with each other, an insulative supporting film formed over one major surface of the substrate so as to cover the through-holes, a fluid temperature measuring resistor formed by a heat-sensitive resistor film deposited at a location of the first through-hole on the supporting film oppositely to the substrate, a heat generating resistor formed of a heat-sensitive resistance film deposited at a location of the second through-hole on the supporting film oppositely to the substrate, an insulative protection film deposited so as to cover the fluid temperature measuring resistor and the heat generating resistor, and a reinforcing film provided for the fluid temperature measuring resistor. The reinforcing film is not provided for the heat generating resistor. Flow rate or alternatively flow speed of a fluid is measured on the basis of phenomenon of heat transfer to the fluid from the heat generating resistor.
By virtue of the structure described above, there can be realized the flow-rate detecting device in which the strength of the diaphragm is increased without degrading the flow detection sensitivity and response performance and which ensures enhanced reliability for the flow-rate detecting operation.
In a preferred mode for carrying out the present invention, the reinforcing film may be deposited at a location of the first through-hole on a surface of the protection film oppositely to the fluid temperature measuring resistor.
Owing to the structure described above, there can be realized the flow-rate detecting device in which the strength of the diaphragm is increased without degrading the flow detection sensitivity and response performance while ensuring high reliability for the flow-rate detecting operation.
In another preferred mode for carrying out the present invention, the reinforcing film may be deposited at a location of the first through-hole on a back surface of the supporting film oppositely to the fluid temperature measuring resistor.
With the structure of the flow-rate detecting device described above, the flow characteristic of the fluid concerned can be stabilized with turbulence being suppressed because the exposed surface of the flow-rate detecting device is formed smoothly.
In yet another preferred mode for carrying out the present invention, the reinforcing film may be formed of a heat-sensitive resistance film deposited around the fluid temperature measuring resistor.
With the structure of the flow-rate detecting device described above, the reinforcing film can be formed simultaneously with the heating resistor and the fluid temperature measuring resistor, whereby the number of the manufacturing steps can be reduced and thus the flow-rate detecting device can be implemented easily and inexpensively.
In still another preferred mode for carrying out the present invention, the reinforcing film may be so formed as to be embedded in the supporting film at a location of the first through-hole.
With the structure of the flow-rate detecting device described above, the material for the reinforcing film can be selected with a high degree of freedom without need for taking into consideration the anti-corrosiveness of the material.
In a further preferred mode for carrying out the present invention, the reinforcing film may be so formed as to be embedded in the protection film at a location of the first through-hole.
With the structure of the flow-rate detecting device described above, the material for the reinforcing film can be selected with a high degree of freedom without considering the anti-corrosiveness.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.